This proposal outlines a strategy for elucidating the role of tumor suppressor genes in the regulation of stem cell niche competition in Drosophila and mouse epithelia. Accomplishing the aims in this proposal will be an important milestone toward our long-term goal of developing and using Drosophila and mouse models of field cancerization to identify targets for early-stage cancer diagnostics and therapeutics. Field cancerization is a feature of many epithelial cancers in which tumors arise from a precancerous field of clonally related, mutant but histologically normal cells. Since precancerous fields form before tumors arise and often remain after tumor resection, diagnostics that identify these fields and therapeutics that target them would have enormous clinical benefit. Yet, despite over 50 years of research, the mechanism of precancerous field formation and type(s) of mutations responsible remain unclear. Our central hypothesis is that field cancerization initiates from a stem cell that acquires a tumor suppressor mutation and spreads to neighboring niches through hyper-competition for stem cell niche occupancy. This model is supported by our studies of epithelial follicle stem cell (FSC) maintenance in the Drosophila ovary and published studies on tumor suppressor phenotypes in mouse epithelia. We found that FSCs compete for niche occupancy with cells produced from neighboring FSCs and we have identified mutations in three tumor suppressors that cause hyper-competition. The homologous mutations in mouse and human epithelia cause a predisposition to cancer. Thus, our hypothesis provides an explanation for how a large region of tissue could have a monoclonal origin and for why these large patches of tissue would be particularly cancer-prone. However, before we can achieve our goal of generating experimental models of field cancerization, it will be essential to (1) identify somatic mutations associated with human cancer that cause hyper-competition in the Drosophila FSC niche and (2) determine whether FSC hyper-competition mutations cause hyper-competition in the mouse intestinal epithelium. To achieve the first aim, we will use mosaic analysis to test 67 Drosophila homologs of human tumor suppressors for FSC hyper-competition phenotypes. To achieve the second aim, we will use a mouse intestinal stem cell specific cre in combination with floxed alleles of Bmpr1a and Dlg1 to generate conditional knockouts in adult mice and assay for stem cell hyper-competition in the intestinal epithelium. This project is significant because it will establish new models for the study of epithelial stem cell competition and will be a critical first step toward the development of animal models of field cancerization. It is an innovative departure from previous studies in that it focuses on understanding conserved mechanisms of field cancerization that will provide a theoretical framework for rational selection of candidate molecular or morphological biomarkers to test on human tissue samples.